1. Field of Invention
The present invention relates generally to the measurement of battery voltages and more particularly to the measurement of multiple voltages from multiple batteries of a battery pack.
2. Status of the Prior Art
In order to properly care for a battery, the voltage of the battery must be monitored precisely in order to determine when to charge the battery and the length of time of any charging. Specifically, when the voltage of the battery drops below a predetermined level, usually the battery needs to be charged. Similarly, during charging, the voltage of the battery should be monitored closely in order to avoid overcharging.
Additionally, the temperature of the battery should be monitored in order to ensure that the temperature of the battery is maintained within a desired range. If the temperature of the battery exceeds a prescribed threshold, the life and the capacity of the battery to maintain a charge can be substantially decreased. Additionally, during charging, if the temperature exceeds the prescribed threshold, the battery may not charge properly or may be permanently damaged.
In large power applications such as those found in electric vehicles, typically, multiple batteries are connected together in order to provide the necessary power. The batteries will be connected together either serially or in parallel as needed by the application and form a battery pack. It is not uncommon for the battery pack to have up to 28 individual batteries.
As will be recognized by those of ordinary skill in the art, it is awkward to measure the voltages of each of the batteries of the battery pack. The voltage measurement must be performed in a timely manner in order to ensure detection of any voltage change and provide compensation. If only one voltage measuring device is used to measure the voltage on multiple batteries, the time to switch the measurement device between the batteries may be relatively long, thereby preventing optimal voltage detection. Furthermore, if a measurement device is provided for each battery, the cost of measuring the voltage increases due to the increased part count. It will be recognized that the total output voltage of the battery pack can also be measured. However, this measurement may result in a very inaccurate reading because it will be the sum of all the voltages of the battery and will not indicate whether an individual battery has been overcharged or undercharged or even if the battery is operating.
During measurement of the battery voltage, it is necessary for the voltage to be stable in order to obtain an accurate measurement. If the voltage is measured directly from the battery during charging or discharging, the voltage may fluctuate greatly due to the load placed on the battery thereby resulting in an inaccurate reading. Accordingly, for a truly accurate reading, the battery should be isolated from the load during voltage measurements.
With the need for vehicles to become more environmentally friendly, automakers are creating hybrid, mini-hybrid, fuel cell and electric vehicles which utilize a battery as an energy storage device. In the electric or fuel cell vehicle, the battery powers electric motors which propel the car. The batteries are charged either through regenerative braking and/or an outside source of electrical power such as a fuel cell. Hybrid and mini-hybrid vehicles do not use an outside source of power to charge the batteries. In hybrid vehicles, a small engine is used to charge the batteries and propel the vehicle. Electric motors are also used to propel the hybrid vehicle. As such, the battery pack is relatively large and provides around 300 volts. Mini-hybrid vehicles use a small battery pack having about 42 volts that is used to power an integrated starter generator of the conventional engine. In both hybrid and mini-hybrid vehicles, the battery is charged via the engine and regenerative braking.
It will be recognized that it is imperative to fully monitor the voltage from the batteries of the electric, fuel cell, hybrid and mini-hybrid vehicles. In order to improve the life expectancy of the batteries, and decrease service and replacement thereof, the voltage of each of the batteries should be closely monitored. Additionally, due to charging via regenerative braking, the batteries are subject to multiple short periods of charging and discharging. Accordingly, the battery voltages must be constantly monitored in order for the batteries to function properly and to prolong the life expectancy thereof.
The present invention addresses the above-mentioned deficiencies in the prior art voltage measurement systems by providing a system and method which accurately measures the voltage of each of the batteries of a battery pack. More specifically, the present invention provides a system which is capable of isolating the voltage of an individual battery during the measurement process. Additionally, the present invention provides a system which measures the voltage of multiple batteries without the need for a measurement device for each individual battery or a single measurement device that measures only the total pack voltage. As such, the present invention provides an efficient voltage measurement system for accurately determining the voltage of each battery in a multiple battery pack.
In accordance with a preferred embodiment of the present invention, there is provided a system for measuring voltages of batteries in a battery pack. The system includes an isolation circuit that is operative to isolate a voltage from each of the batteries and a buffer in electrical communication with the isolation circuit. The buffer is operative to store the voltage of each of the batteries when the batteries are isolated. A voltage measuring device of the system is in electrical communication with the buffer. The voltage measuring device is operative to measure the voltages in the buffer in order to provide a stable measurement of the battery voltages.
In the preferred embodiment of the present invention, each battery will have its own isolation circuit. The battery pack will be configured into multiple banks of batteries and the system will have a buffer for each battery of each bank.
In this regard, each buffer will have a number of inputs corresponding to the number of banks of batteries. Similarly, the system will include a voltage measuring device for each of the buffers such that the number of buffers will equal the number of voltage measuring devices.
In accordance with the present invention, the system will further include a controller (i.e., microcontroller or microprocessor) operative to control the isolation circuits, the buffers and the voltage measuring devices. The controller operates the components such that the battery voltages for each bank of batteries will be sequentially determined. The controller may also be operative to monitor the temperature of the battery pack via a temperature sensor. In the preferred embodiment, the system will contain twelve temperature sensors.
It will be recognized that each isolation circuit may contain a relay to isolate the battery from the buffer. Disposed between the relay and the buffer will be an energy storage device operative to store the voltage of the battery after the battery has been isolated from the buffer. In this regard, the energy storage device may be a capacitor.
In accordance with the present invention, the measurement system may measure the voltage of l batteries configured in m battery banks. Each battery bank has n batteries. The system will then have l isolation circuits for each of the l batteries. Furthermore, the system will have n buffers operative to store the voltage of the batteries when isolated. Finally, the system will have n voltage measuring devices for each of the n buffers. In the preferred embodiment, there will be a total of twenty-eight batteries configured into four banks such that each bank will have seven batteries. Accordingly, the system will contain 28 isolation circuits, seven buffers and seven voltage measuring devices.
In accordance with the present invention, there is provided a method of measuring the voltages of batteries in a battery pack. The method begins by storing the voltage of a respective one of the batteries in an energy storage device such as a capacitor. Next, the battery is isolated from the energy storage device and the voltage is stored in a buffer. Finally, the voltage is measured from the buffer in order to provide a stable voltage measurement for the battery.
These, as well as other features of the present invention will become more apparent upon reference to the drawings wherein:
FIG. 1 is a schematic illustration of a voltage monitoring system constructed in accordance with the preferred embodiment of the present invention; and
FIG. 2 is an exploded view of a section of the voltage monitoring system shown in FIG. 1.